The present invention relates to video reproducing or playback apparatus, and more particularly to such apparatus which is capable of obtaining a high quality of reproduced image without noise bars at a high reproduction speed.
High speed reproduction in a video tape recorder (hereinafter referred to as "VTR") using a field memory will be described where it is driven at an even numbered multiple of the recording speed i.e. m times normal speed, where m is an even number and equal to, for example, four.
Generally, an odd numbered multiple or n times normal speed, wherein is an odd number, is used for high speed reproduction so as to utilize the property that the positions of noise bars are fixed due to the fact that the position of the noise of every field is the same. On the contrary, when the signal reproduction is conducted at m normal speed, the positions of the noise and the signal are replaced by each other at every field. If this property is utilized in using a memory, it becomes possible to narrow the noise bar.
FIG. 1 is illustrative of a high speed playback system of a typical prior art VTR. In FIG. 1, the reference numeral 1 designates a video tape onto which signals are recorded. The signals reproduced from the video tape 1 by the video heads 2a and 2b are applied to the preamplifier 3, and thereafter, the reproduced signals are fed to the video signal processing circuit 4. Reference numeral 5 designates an envelope detector for detecting the envelope of the reproduced signal from the output of the preamplifier 3. The output of the envelope detector 5 is connected to the comparator 6 which compares the same with a predetermined level, and the output thereof is coupled to the memory control circuit 7 which generates timing and the address signals for the writing in of the output signal which is output from the video signal processing circuit 4 to a field memory 8. A signal is sent to the memory control circuit 7 from the video signal processing circuit 4.
The field memory 8 is a dualport memory or a multiport memory (not shown). This memory has a random output port and a serial output port as output ports; and if the serial port is used, the writing in and the reading out from the memory can be conducted asynchronously. The operations herewith are conducted asynchronously in such a manner that the reproduced signal from the video signal processing circuit 4 is written in into the field memory 8 and the content of the field memory 8 is then read out with the use of the serial port.
Further, reference numeral 9 designates a control head, and on the basis of the output of the control head 9 the servo circuit 10 conducts the running control of the tape in each operational mode by controlling the capstan motor 11 and the reel motor 12.
The device operates as follows.
Assume now that a high speed reproduction of playback of the video tape 1 is provided at a speed of four times (m=4) the normal speed in the reverse direction. FIGS. 2 and 3 are diagrams for exemplifying such operation. In FIG. 2, reference numeral 50 designates a video track, and reference characters A and B represent the azimuth recordings. The video head 2a (FIG. 1) has the same azimuth as A and the video head 2b has the same azimuth as B, respectively.
When the video head 2a traces the locus of the dotted lines d in the drawing, the reproduced signal, that is, the output of the preamplifier 3 appears as shown in FIG. 3(a). Similarly as above, when the video head 2b traces the locus of the dotted lines e in the drawing, an output as shown in FIG. 3(b) is obtained. When the contents of the two fields as illustrated in FIGS. 3(a) and 3(b) are mutually interlaced on the time axis, a video image of one field as shown in FIGS. 3(c) and 3(c') is obtained, and this is stored in the field memory 8 and displayed on a monitor. Herein, the envelope waveforms shown in FIGS. 3(a), 3(b), 3(c), and 3(c') indicate only the upper portions of the alternate waveform which has symmetrical configurations at the upper and lower portions.
Such a one field content having substantially no noise bar is obtained in a case where the width of the video track is approximately equal to or larger than the width of the magnetic head with no guardband. Actually, in apparatus having a head construction which utilizes both the standard mode and the triple speed mode in a VHS system, the construction of the head is mainly directed to a triple speed mode use.
When a standard mode recording is conducted in such apparatus, a portion of 1/2 to 2/3 of the video track 50 becomes a guardband, and the video track becomes as shown by reference numeral 51 in FIG. 4. The output of the preamplifier 3, obtained by tracing the loci of the dotted lines f and g by the respective video heads 2a and 2b on the video track 51, becomes as shown in FIGS. 5(a) and 5(b), respectively, and one field video image is obtained when these are mutually interpolated on the time axis becomes as shown in FIG. 5(c).
Accordingly, when the video image is interlaced on the time axis shown in FIGS. 3(c) and 3(c'), and the waveform of FIG. 5(c) is written in into the field memory 8, the comparison voltage of the comparator 6 must be changed. That is, in FIG. 3(c), if the comparison voltage of the comparator 6 comprises the level of S1, only that portion for which a signal is obtained is written in into the field memory 8 at every field. This is an ideal state, and the composite video information interlaced within one field period continue smoothly. On the other hand, if the level is at the level S2, a reproduced signal is not obtained for the portion 0 and appears as a noise region. Similarly, if the level is fixed to the level S3 the content of the portion P is rewritten at every field, and it appears as a blur. Furthermore, if the comparison voltage of the comparator 6 is made a "0" level in FIG. 5(c), almost all of the envelope is written in into the field memory 8. Herein, although the signal to noise ratio or S/N is low and there appears a noise region having a noise width of some degree at the portion designated by l, the interlaced video information continues smoothly thereat because it is almost impossible to obtain a signal information thereat.
In FIG. 3(c'), however, when zero voltage is applied to the comparator 6 similarly as above, the portion designated by J appears as a blur because the content thereof is rewritten at every field, and the noise which arises accompanying the deterioration of S/N appear in the vicinity of the both ends m and n of J. That is, two noise bars having noise widths of some degree appear at a portion where only a noise bar having quite a small width (one horizontal scanning period) originally appeared. That is, in FIGS. 2 and 4 the recording track width on the tape are different from each other, and if the same comparison reference voltage is used, the number and the noise width of the noise bars will have different values.
As described above, in a device which effects high speed reproduction at m times normal speed, where m is an even number, with the use of a field memory and provides interlacing of the content of each field to obtain signal information, a problems arises in that the number of noise bars is increased and the width of the noise bars is widened dependent on the level variation of the reproduced signal which is reproduced from the tape or is widened dependent on the recording track width on the tape.